The present invention relates to a process for the detection and recording of defects and/or weak points on hot semifinished iron and steel industry products.
Iron and steel industry semifinished products such as slabs, blooms, steel rods, which are obtained from ingot casting or from continuous casting, are generally transformed into flat products by hot rolling.
In an action allowing energy savings and decreased production costs, the iron and steel industry seeks maximum benefit from the heat of casting of semifinished products, to effect the subsequent rolling operation. However, the semifinished products can have visible or invisible defects or weak points, in the vicinity of their surfaces, which are fissures, tears, folds or some other defect type, which risk remaining on the rolled products or even being amplified and causing rejection of the piece, or even damage of the surface of the roller cylinders. That is why it has proven necessary to examine the semifinished products before proceeding with rolling, to determine whether they can be rolled directly or whether they must be treated beforehand to delete or repair the defects which they have, or even if they are of such low quality that they must be recycled.
Until this time, because of the processes and materials used, this examination for the detection of the weak points or defects required precooling of the semifinished products; this led the iron and steel industry to seek ways to do this and to save energy. Studies have consequently been undertaken by the iron and steel industry in Europe, the United States and Japan to find a method which will allow detection and repair of the weak points or defects in the product, the method being such that it can where desired be introduced into the chain of casting and rolling operations, so as to realize an entirely continuous process.
Because the semifinished products are hot, the observer cannot examine them directly. All of the studies undertaken until this time have therefore been based on the principle of forming an image of the weak points or defects from a distance, which will then serve as guide for the automatic repair. To apply this principle, an entire series of more or less sophisticated processes have been designed, which differ both in the means of obtaining the image and in the means of recording this image and using this recording to effect the repair of the semifinished products.
But these processes, besides the fact that they require a long and difficult installation, generally present drawbacks, particularly in the detection of the weak points or defects. In fact, the image which is obtained is not the reflection of what an observer might see, but most often is a collection of points obtained from the physical features which are expected to represent the "health" of the semifinished products. Because of this, it is practically impossible for the observer to distinguish between the points which actually correspond to the state of the semifinished products and the points resulting from disturbance of the transmission of signals issuing from these features; the faculty of interpretation and the judgment of the observer is thus practically inconsequential. The recording of such images and the use of such images by the repairer can then lead to nonuniform semifinished products being fed into rolling.
Each of these processes also presents specific deficiencies. Thus, for example, according to the type of detection which is used, it is observed that:
The use of a camera and infrared television gives images disturbed by the presence of powders which are for lubrication of the ingot molds, oxidation spots, and irregularities of surface due to oscillation of the ingot molds, which makes analysis difficult. Moreover, these systems give no indication at all of the depth of the defects or weak points.
The use of television and Foucault currents allow visualization of the weak points or defects in depth but along only short lengths (1 to 5 mm).
The electromagnetic and ultrasonic systems are sensitive to metallurgical transformations in the vicinity of the Curie point (600 to 700 degrees C.) which disturb the transmission of waves and lead to parasitic effects. They also require deoxidation beforehand of the surfce of the semifinished products to allow a suitable transducer-steel coupling.
The induction preheating and infrared detection systems give results which are controlled by the angle of the defect in relation to the measurement axis; transverse defects or defects which present angles less than 15 degrees in relation to the transverse axis are poorly detected.